Carbon-containing black glass monoliths

ABSTRACT

Carbon-containing black glass compositions of matter having the empirical formula SiC x  O y  and derived from the polymer precursors having the residue ##STR1##

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 07/816,269, filedDec. 23, 1991 now U.S. Pat. No. 5,328,976 which is continuation-in-partof U.S. Ser. No. 07/002,049, filed Jan. 9, 1987 now U.S. Pat. No.5,242,866.

BACKGROUND OF THE INVENTION

Ceramics have been known for many hundreds of years and have been usedas coatings or as fabricated parts and are employed wherever theircharacteristics such as durability, nonporosity, electrical conductivityor nonconductivity, and heat protection are required. One of the morerecent ceramic materials is a silicon-carbon-oxygen system, named as ablack glass, which can find use in certain situations where extremelyhigh temperatures are present.

Traditionally, the introduction of carbon in glasses was made byimpregnating porous glass with a concentrated solution of an organiccompound and subsequently firing in a reducing or neutral atmosphere.The carbon-containing product is generally regarded as a compositecontaining carbon and silica. Elmer and Meissner (Journal of theAmerican Ceramic Society, 59, 206, 1976) of Corning Glass Works reportedthat the annealing point of reconstructed 96% silicon dioxide glasses ismarkedly increased by incorporating carbon in porous glass. Furfurylalcohol was used as the pyrolyzable organic compound. They attributedthe increase of about 100° C. in annealing point to the effect ofhydroxyl removal from the internal surface of the porous glass byhydroxyl reaction with carbon. The resistivities of samples with lessthan 2% carbon content approached that of the glass whereas theelectrical resistivities of carbon-containing silica with carbon between4.5-7% are in the range of 1-3 ohm-cm, thus producing electricallyconductive glasses. The highest carbon content in the final glasses theycould produce is 8.59%.

Smith & Crandall reported in U.S. Pat. No. 3,378,431 a method of makingcarbon-containing glass by hot-pressing to sintering temperature amixture of colloidal silica and an organic compound known in the tradeas "Carbowax" (polyethylene glycol) in an oxygen-free atmosphere. Theblack glass obtained from the mixture of 33% "Carbowax" and 67% silicondioxide showed the presence of 1.2% by weight of carbon. Adevitrification-resistant bonded mass of vitreous silicon dioxide andcarbon physically inseparable and microscopically indistinguishable fromsilica was obtained. The black glass has a low thermal diffusivity andmore resistance to crystallization than pure vitreous silica.Devitrification temperature increased by 150° C. to 1250° C. as comparedwith colloidal silica.

Carbon-modified silica glass has been used as a composite matrix byLarsen, Harada and Nakamum (Report No. AFWAS-TR-83-4134, December, 1983,Wright-Patterson AFB, Ohio). In producing fiber-reinforced composites,the processing sequence includes slurry impregnation of silicon carbidefiber in an aqueous slurry of a carbowax (polyethylene glycol) and asilicon-containing compound known in the trade as Cab-O-Sil (a silicondioxide powder manufactured by Cabot), layout of prepregged fiber tows,and hot-pressing. The composites thus obtained exhibited high porosityand brittle fracture indicative of low toughness. They concluded thatthe silicon carbide/black glass fiber composite is a promising material,although the property goals were not achieved. There is suspicion thatthe silicon carbide fibers may have degraded.

More recently, formation of carbonaceous ceramics has been carried outthrough the use of the sol-gel process. January discloses in U.S. Pat.No. 4,472,510 the use of the sol gel process to form monolithic glassescontaining carbon through pyrolysis of the gels oforganosilsesquioxanes, metal oxides and metal alkoxides. Monomann inGreat Britain Patent 1,359,576 disclosed the formation of silicon andquartz fibers using silsesquioxanes as precursors. Their gelling processused selected organosilicon compounds for the preparation of the ceramicprecursor based on the following reaction:

    .tbd.Si-OR+H.sub.2 O→.tbd.Si-OH+ROH                 (1)

    .tbd.Si-OH+HO-Si.tbd.→.tbd.Si-O-Si.tbd.+H.sub.2 O   (2)

in which R represents an organic radical such as alkyl groups and arylgroups such as phenyl group.

The uniqueness of the sol-gel process is the ability to obtainhomogeneous, purer glassy products by low temperature processes. Also,the use of a liquid sol as the starting material allows the preparationof intractable monoliths of complicated shapes utilizing a liquid path.The advantages of such a procedure over the powder consolidationtechniques, such as sintering and hot isostatic pressing, are theirformability of complicated shapes and low temperature operation.However, monolithic black glasses produced via hydrolysis andcondensation of organoalkoxysilanes are not practical because of therequirement for very long drying periods and delicate gellingconditions. For example, January prepared a 0.66 cubic centimetermethyltrimethoxysilane gel monolith over a drying period of about threeweeks, which, upon pyrolysis, yielded a carbon-containing black glassmonolith of density 1.6 grams per milliliter.

The very slow drying rate is necessary for reducing cracks during thegelation period. These cracks form as a result of the non-uniformsurface tensions created by the evaporation of the split-off water oralcohol molecules in the hydrolysis (1) and condensation (2) reactions.

Silsesquioxanes have also been produced by titanium catalyzedredistribution of methylhydridooligo-and-polysiloxanes by R. M. Laine etal., in Chem. Mater. 1990, 2, 464-472. A gaseous by-product (methylsilane, b.p. -57° C.) is produced during the redistribution reaction.

In the instant invention, a hydrosilylation reaction was used for thegelation process in place of the hydrolysis-condensation route. Thehydrosilylation involves addition of silane (Si-H) to vinyl silane(Si-CH=CH₂) to form an ethylene or methylene linkage as illustrated inthe following equations: ##STR2## The features of the hydrosilylationreaction are such that there is neither a small molecule reactionproduct nor a weight loss during gelation and that the carbons in theethylene linkage are bonded to the silicon atoms. This gelation reactioncompletely eliminates the drying problem inherent in the hydrolysis oforganoalkoxysilane process. We also found that cyclosiloxane gelscross-linked by hydrosilylation reaction produced upon pyrolysis to hightemperature in a non-oxidizing atmosphere high carbon content, highyield and high density black glasses.

N. Harada and M. Tanaka in U.S. Pat. No. 3,957,717 described and claimedan organopolysiloxane gel prepared from cyclosiloxanes and H. Lamoreauxin U.S. Pat. Nos. 3,197,432 and 3,197,433 claimed the product gel fromreacting cyclosiloxanes containing hydrogens and vinyl groups. The basicidea of reacting silyl hydrogen groups with silyl vinyl groups is foundin U.S. Pat. Nos. 3,439,014 and 3,271,362.

Monomann in Great Britain Patent 1,359,576 disclosed the use of a phenylgroup rather than a methyl group as R in order to increase the carboncontent of their products. By choosing phenyl group as R, the carbonweight percent can be increased to as high as ca. 30%. However, we haveshown in our simulation experiments that the carbon present started tooxidize at 550° C. in flowing air and was completely removed before1000° C. Therefore, the carbon derived from pyrolysis of the phenylgroup is free carbon susceptible to oxidation while our inventionresults in a carbon-containing material that is oxidation resistant upto about 1400° C.

Okamura et al. reported in U.S. Pat. No. 4,618,591 a method of makingsilicon carbide-carbon composite molded product by using polycarbosilaneas the precursor for a matrix material. The polycarbosilane on pyrolysisforms microcrystalline silicon carbide with inclusion of low oxygenpercentage, as indicated by their X-ray diffraction patterns. Incontradistinction to this work, this invention produces materials thathave different composition ranges and that are overwhelmingly amorphouswith a few small diffraction peaks different from silicon carbide.

The stability of a soluble polymer was studied by thermogravimetricanalysis by A. Zhdanov et al. and reported in the Russian JournalVysokomolekulyarnye Soedineniva, Series A, 16 (10), 2345-50 (1974). Theyprecipitated the highly branched, soluble polymer from the reactionmixture as powders by adding alcohols into the reaction vessel beforethe gel point. Their polymer was different from a network gel producedfrom a sol-gel process in that it contained a large amount of unreactedSi-H and Si-CH=CH₂ groups and was readily soluble in aromatic solvents.Also, the polymer powder did not melt when heated up to 500° C. Theyheated the soluble polymers at 10° C. per minute up to a maximum of 780°C. in both Argon and air and reported the thermogravimetric results asto weight loss at various stages of heating and as to the total weightloss involved. No weight change was observed beyond 780° C. when heatedin Argon at a rate of 10° C./min. with a final yield of 87%. TheRussians did not characterize the resultant product of this analysis andappeared to have no interest in this product.

In contradistinction to this prior work, this invention is concernedwith the product of pyrolysis of the gel polymers formed fromcyclosiloxanes as well as with the process to produce such a product.The product of our invention is a hard, glassy material which we call ablack glass having oxidation-resistant carbon and which is very usefulwhen cast as a monolith, or one piece object.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a composition of matter in which greateramounts of carbon are incorporated by bonding to silicon than werepossible utilizing prior art. More specifically, the invention isconcerned with a carbon-containing black glass composition of matter inwhich up to about 40% carbon is incorporated by weight to produce anoxidatively stable and high melting substance.

As was hereinbefore discussed, there is a need for a thermally stable,oxidative-resistant, and devitrification-resistant black glass. Such amaterial would find high temperature use and would be economicallyattractive when prepared by the present method in which a polymer wouldbe formed at a low temperature followed by pyrolysis at temperatures inthe range of about 700° C. to about 1400° C. Our invention has theadvantage of producing a silica-modified glass having a higher meltingpoint than cristobalite and having greater resistance to devitrificationthan pure vitreous silica and previously known carbon-containingglasses. Our invention also yields a carbon-containing glass havinghigher thermal stability in air than known nonoxide ceramics containingcarbon.

In one aspect, an embodiment of this invention resides in acarbon-containing black glass ceramic composition of matter having theempirical formula SiC_(x) O_(y) wherein x is greater than zero and up toabout 2.0, preferably up to about 1.6 and y is greater than zero and upto about 3.2, preferably up to about 1.8.

Another aspect of this invention is found in a process to produce ablack glass comprising making a polymer by reacting, in the presence ofa catalytic effective amount of a hydrosilylation catalyst, (a) acyclosiloxane monomer of formula ##STR3## where n is an integer from 3to about 20, R is hydrogen, and R' is an alkene of from 2 to about 20carbon atoms in which one vinyl carbon is directly bonded to silicon or(b) two or more different cyclosiloxane monomers of the formula of (a)where for at least one monomer R is hydrogen and R' is an alkyl grouphaving from 1 to about 20 carbon atoms, and for the other monomers R isan alkene of from 2 to about 20 carbon atoms in which one vinyl carbonis directly bonded to silicon and R' is an alkyl group of from 1 toabout 20 carbon atoms, or (c) cyclosiloxane monomers having the formulaof (a) where R and R' are independently selected from hydrogen, analkene of from 2 to about 20 carbon atoms in which one vinyl carbon atomis directly bonded to silicon, or an alkyl group of from 1 to about 20carbon atoms and at least one of said monomers contains each of saidhydrogen, alkene, and alkyl moieties, heating the resulting polymer in anon-oxidizing atmosphere to a temperature in the range of from about800° C. to about 1400° C. or in an oxidizing atmosphere at a rateexceeding 5° C./min to the same temperature range, to produce a blackglass.

In another aspect, the invention relates to a black glass produced byheating a polymer containing the moieties ##STR4## where

R^(o) is the unreacted residue of an alkene having 2 to 20 carbon atoms

R" is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having2 to 20 carbon atoms

in a non-oxidizing atmosphere to a temperature in the range of fromabout 800° C. to about 1400° C. to produce black glass, or in anoxidizing atmosphere at a rate exceeding 5° C./min, preferably 50°C./min to 1000° C./min most preferably greater than 100° C./min, to thesame temperature range.

In still another aspect the polymer moieties are derived from thereaction of cyclosiloxane monomers containing silicon-vinyl bonds and/orsilicon-hydride bonds or the reaction of cyclosiloxane monomerscontaining silicon-hydride bonds with acetylene, or the reaction ofcyclosiloxane monomers containing two hydride bonds attached to asilicon atom with cyclosiloxane monomers containing silicon-vinyl bonds.

DETAILED DESCRIPTION OF THE INVENTION

Precursor Polymers

The black glass of the invention is derived from cyclosiloxane polymerprecursors. In one embodiment, the precursor polymers are prepared bythe reaction of cyclosiloxanes containing a silicon-hydride bond and/ora silicon-vinyl bond, as defined below. For example, such polymers maybe described as the reaction product of (a) a cyclosiloxane monomer ofthe formula ##STR5## where n is an integer from 3 to about 20, R ishydrogen, and R' is an alkene of from 2 to about 20 carbon atoms inwhich one vinyl carbon is directly bonded to silicon or (b) two or moredifferent cyclosiloxane monomers of the formula of (a) where for atleast one monomer R is hydrogen and R' is an alkyl group having from 1to about 20 carbon atoms, and for the other monomers R is an alkene offrom 2 to about 20 carbon atoms in which one vinyl carbon is directlybonded to silicon and R' is an alkyl group of from 1 to about 20 carbonatoms, or (c) cyclosiloxane monomers having the formula of (a) where Rand R' are independently selected from hydrogen, an alkene of from 2 toabout 20 carbon atoms, in which one vinyl carbon atom is directly bondedto silicon, or an alkyl group of from 1 to about 20 carbon atoms and atleast some of said monomers contain each of said hydrogen, alkene, andalkyl moieties, said reaction taking place in the presence of aneffective amount of a hydrosilylation catalyst.

Other methods of preparing such cyclosiloxane polymers may be employed.For example, the cyclosiloxane monomer defined above may contain onlyhydrogen atoms rather than vinyl or alkyl groups, that is R and R' areonly hydrogen. Such a cyclosiloxane could be reacted with acetylene orsubstituted acetylenes to provide the same linking moiety between twocyclosiloxane groups. This could be accomplished at temperatures ofabout 25° to 200° C. and pressures of about 1 to 14,000 kPa usingplatinum, chloroplatinic acid, tris(triphenylphosphine) Rhodium (I)chloride, or tris(triphenyl phosphine) Ruthenium (II) chloride as acatalyst. Alternatively, a cyclosiloxane containing a single hydrogenatom and a methyl group attached to the silicon atom could be reactedwith acetylene in a similar manner.

Another procedure related to the methods described above is the reactionof a cyclosiloxane containing only hydrogen atoms attached to a siliconatom with cyclosiloxanes containing vinyl groups attached to the siliconatoms.

The objective of the methods described and others which may occur tothose skilled in the art is to provide polymers containing the moieties##STR6## where

R^(o) is the unreacted residue of an alkene having 2 to 20 carbon atoms

R" is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having2 to 20 carbon atoms

Since the cyclosiloxane molecules are linked by the reaction of ahydrogen atom from one molecule and an alkene from the other, theresidual group R^(o) could result from species of R and R' previouslydescribed. Thus, R^(o) could be hydrogen or an alkyl group of 1 to 18carbon atoms or an alkene group of 1 to 18 carbon atoms containingunsaturation in addition to the reactive vinyl group. It is believedthat the improved high temperature performance of black glass of theinvention is related to the relatively close coupling of the silicon andcarbon atoms.

In a preferred embodiment the polymer precursor of the present inventionmay be prepared by heating a mixture of cyclosiloxane monomerscontaining silicon-hydride and silicon-vinyl bonds to a temperature inthe range of from about 10° C. to about 300° C. in the presence of aplatinum hydrosilylation catalyst present at 1-200 ppm for a time in therange of from about 1 minute to about 600 minutes. The resulting polymeris converted to black glass by pyrolyzing in an inert atmosphere at atemperature in the range from about 800° C. to about 1400° C. for a timein the range of from about 1 hour to about 300 hours, or by rapidheating in an oxidizing atmosphere. The polymer formation step from themonomer takes advantage of the fact that a silicon-hydride will reactwith a silicon-vinyl group to form a silicon-carbon-carbon-silicon or asilicon-carbon-silicon bonded chain, thereby forming a network polymer.Thus, the reacting monomeric cyclosiloxanes contain either asilicon-hydride bond or a silicon-vinyl bond or both. A "silicon-hydridebond" refers to the presence of a silicon atom bonded directly to ahydrogen atom and a "silicon-vinyl bond" refers to the presence of asilicon atom bonded directly to an alkene carbon atom i.e. an atom whichis attached by a double bond to an adjacent carbon atom.

The pyrolysis chemistry of the gel polymer in this invention isdistinctly different from that reported by A. Zhdanov et al. in thattheir soluble polymer did not have any reaction above 780° C. in a fastheating of 600° C. per hour. As discussed hereinbefore, this solublecyclosiloxane precursor is also chemically different from a gel polymer.The gel polymers in this invention cannot be dissolved in solvents suchas toluene. In contrast, conversion of the gel polymer to black glass byheating in an inert atmosphere takes place between 430° C. and 950° C.Three major pyrolysis steps were identified by thermogravimetricanalysis at 430°-700° C., 680°-800° C. and 780°-950° C. The yield of thegel-glass conversion is 83%; the third pyrolysis mechanism occurringbetween 780° C. and 950° C. contributed the final 2.5% weight loss tothe final product.

The invention can be practiced by utilizing a polymer precursorcyclosiloxane wherein both the requisite silicon-hydride bond and thesilicon-vinyl bond are present in one molecule. For example,1,3,5,7-tetravinyl-, 1,3,5,7-tetrahydrocyclotetrasiloxane would operatewithin the scope of this invention since this molecule has the basicrequirement of a silicon-hydride bond and a silicon-vinyl bond and wouldpolymerize to give a black glass polymer precursor of use in thisinvention.

Platinum is the preferred hydrosilylation catalyst. Other catalysts suchas cobalt and manganese carbonyl will perform adequately. The catalystcan be dispersed as a solid or can be used as a solution when added tothe cyclosiloxane monomer.

Cyclosiloxanes are the preferred silicon containing compounds foreffecting the gel monoliths. Examples of cyclosiloxanes include, but arenot limited to,

1,3,5,7-tetramethyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetravinyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetravinyltetraethylcyclotetrasiloxane,

1,3,5,7-tetravinyltetramethylcyclotetrasiloxane,

1,3,5-trimethyltrivinylcyclotrisiloxane,

1,3,5-trivinyltrihydrocyclotrisiloxane,

1,3,5-trimethyltrihydrocyclotrisiloxane,

1,3,5,7,9-pentavinylpentahydrocyclopentasiloxane,

1,3,5,7,9-pentavinylpentamethylcyclopentasiloxane,

1,1,3,3,5,5,7,7-octavinylcyclotetrasiloxane,

1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane,

1,3,5,7,9,11-hexavinylhexamethylcyclohexasiloxane,

1,3,5,7,9,11-hexamethylhexahydrocyclohexasiloxane,

1,3,5,7,9,11,13,15,17,19-decavinyldecahydrocyclodecasiloxane,

1,3,5,7,9,11,13,15,17,19,21,23,25,27,29-pentadecavinylpentadecahydrocyclopentadecasiloxane,

1,3,5,7-tetrapropenyltetrahydrocyclotetrasiloxane,

1,3,5,7-tetrapentenyltetrapentylcyclotetrasiloxane and

1,3,5,7,9-pentadecenylpentapropylcyclopentasiloxane.

1,3,5,7,9-pentahydropentamethylcyclopentasiloxane

1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane

1,1,3,3,5,5,7,7,9,9,-decahydrocyclopentasiloxane

1,1,3,3,5,5,7,7,9,9,11,11-dodecahydrocyclohexasiloxane

From a broader perspective, the black glass of the invention may bederived from polymers which contain he moieties ##STR7## where

R^(o) is the unreacted residue of an alkene having 2 to 20 carbon atoms

R" is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having2 to 20 carbon atoms

The polymers described by the reaction of cyclosiloxanes havingsilicon-hydride bonds with those having silicon-vinyl bonds will containresidues of these types. Thus the reaction of various types ofcyclosiloxanes could produce polymers of the invention, as alreadysuggested. In addition, the linking of cyclosiloxane monomers by thereaction of acetylene with silicon-hydride moieties could produce suchpolymers. Other reactions which result in polymers having these linkagesbetween cyclosiloxane molecules would be expected to provide precursorsto black glass of the invention.

Producing Black Glass

One of the most useful methods utilized in the process of this inventionis to fabricate the polymer precursor into a monolith using procedureslike tape casting, injection molding, reaction injection molding, andcompression molding. For instance, the polymer forming cyclosiloxanemixture may be introduced into a mold and then heated to form thepolymer monolith black glass precursor or extruded through a heated dieto form a precursor polymer monolith. The monolith would then bepyrolyzed up to about 1400° C. to form the black glass monolith.

Also considered as within the scope of this invention is impregnatingthe black glass product of this invention with cyclosiloxane monomerreaction mixture, the best results coming from pressure or vacuumimpregnation with subsequent pyrolysis to afford a black glass productwith less cracks and voids and with greater density. Impregnation can berepeated to further increase the density of the black glass product ofthis invention.

The monomeric mixture can include a filler such as cubic or hexagonalsilicon carbide, silicon nitride, silica, alumina, hafnia, titania, andzirconia to strengthen the resulting monolith. Such a filler in the formof a powder, whisker, or fiber can be mixed into the monomer usingconventional means. The filled product produced by the process of thisinvention shows not only increased strength but also exhibits controlledshrinkage upon the pyrolysis step. Pyrolysis of the precursor polymerschanges these polymers into very hard ceramic bodies that can findapplication in high temperature, oxidation-resistant, high strengthcomposite matrices and castable ceramics.

It has been discovered that application of pressure to the monomermixture during the polymer forming operation will prevent nucleatebubbling of the reactants and decrease reaction time in that highertemperatures can be employed. Bubbling is to be avoided since it causesvoids and cracks to form in the incipient polymer and thereby weakeningthe finished product. In the reaction of this invention bubbling occurswhenever the filler content of the monomer mixture is in excess of about20% by weight. Therefore, it is preferred to perform the polymerizationof this invention under a pressure in the range of from 14 psi to about30,000 psi so as to produce crack-free nearly voidless polymers. Theapplication of increased pressure will also hasten the reaction time forunfilled polymer formation. The term "crack-free" should be read to meanfree of visible cracks.

Harada and Tanaka have shown in their control experiment that the curedproduct obtained from a mixture of 175 parts of cyclotetrasiloxanes and200 parts of quartz flour was found to have cracks and to be unusable.In their invention, 100 parts of an organopolysiloxane composed of thetriorganosiloxy and silicon dioxide groups was added to thecyclotetrasiloxanes sol, resulting in a cured product free from crackdefects. Their monomer composition thus prepared is curable at atemperature in the range from room temperature to 100° C. Our inventionof high pressure polymerization of cyclosiloxanes by hydrosilylationreaction not only can produce cured thick-walled products withoutcracking and gas pockets, but also allows the use of polymerizationtemperatures higher than 100° C. and higher platinum concentration, thusshortening the reaction time for polymerization.

The black glass composition of matter has an empirical formula SiC_(x)O_(y) wherein x is greater than zero and up to about 2.0, preferably upto about 1.6, and y is greater than zero and up to about 3.0, up toabout 1.8. The carbon content ranges up to about 40%. No other methodknown in the art can achieve such a high carbon content black glasswherein the carbon is resistant to oxidation at high temperatures.

As discussed herein before, January and Monomann were able to producehigh carbon black glasses from precursors different from this inventionbut their glass contained low densities around 1.6 and the carbon waseasily oxidized at low temperature. Using the process of our invention,the carbon contained in the black glass is resistant to oxidation andour densities are about 2.1 grams per milliliter. In addition, the priorwork utilizing silicon hydrolysis had extremely slow fabrication timesfor monoliths on the order of weeks, whereas our invention can form thepolymer monoliths in the order of minutes with higher yield than thosemade from hydrolysis reactions of silicon. Our monoliths can be formedinto larger shapes than the hydrolysis black glasses.

Our invention can be used to manufacture non-porous as well as porousblack glass. For most purposes it is preferred to use neatcyclosiloxanes to form non-porous black glass, but porous black glasscan be formed if so desired by starting with solvent based cyclosiloxanemonomers. The use of pressure will provide crack-free polymers where thesame reaction mixture run at atmospheric mixture provides a polymercontaining cracks.

EXAMPLE I

Ten milliliters of tetravinyltetramethylcyclotetrasiloxane was mixedwith 7.2 milliliters of a mixture of cyclosiloxanes containing from 3 toabout 6 silicon atoms and called methylhydrocyclosiloxane and 0.05milliliters of platinum-divinyltetramethyldisiloxane complex containing3% platinum in xylene was added to the above mixture. After heating toabout 60° C. for one hour a toluene insoluble gel polymer (Polymer A)was formed. The resultant polymer was then pyrolyzed in nitrogen at aheating rate of 200° C. per hour to about 1200° C. resulting information of a carbon containing black glass. The weight loss was about17% for the overall process and the skeleton density for the groundblack glass powder was about 2.10 grams per milliliter. Thecarbon-containing black glass lost less than 0.6% by weight when heatedin flowing air to about 1150° C. at a heating rate of 10° C. per minutein a thermogravimetric analysis. X-ray analysis of this black glassindicates that this material is largely amorphous and that the samplehad a few small diffraction peaks, which was different from crystallinesilicon carbide. Elemental composition gave the formula

    SiC.sub.1.37 O.sub.1.03

for the black glass and the black glass contained <0.1 weight % hydrogenand <0.3 weight % nitrogen.

EXAMPLES II-VIII

These examples were all performed as described in Example I with theexception that different volume amounts oftetravinyltetramethylcyclotetrasiloxane (T) and methylhydrocyclosiloxane(M) were utilized to make the black glass. The results of theseexperiments are presented in Table 1 below where T/M is a volume ratio.The data shows that the silicon bonded carbon content can be varied andthis variation is controllable within ±1%. Thermogravimetric analysis inflowing air showed that powder samples from Examples II-VIII had lessthan 0.5% weight loss when heated to 1150° C.

                  TABLE 1                                                         ______________________________________                                        Composition Tables                                                                            Pyrolysis Carbon wt. %                                                                             Empirical                                Example T/M     Yield     in black glass                                                                           Formulae                                 ______________________________________                                        II      8/2     67%       29.8%      SiC.sub.1.45 O.sub.0.89                  III     7/3     79%       28.0%      SiC.sub.1.51 O.sub.1.17                  IV      6/4     82%       27.2%      SiC.sub.1.36 O.sub.0.98                  V       5/5     83%       24.6%      SiC.sub.1.30 O.sub.0.95                  VI      4/6     84%       24.1%      SiC.sub.1.23 O.sub.1.16                  VII     3/7     77%       21.7%      SiC.sub.1.08 O.sub.1.17                  VIII    2/8     57%       19.4%      SiC.sub.1.01 O.sub.1.39                  ______________________________________                                    

EXAMPLE IX

The pyrolysis mechanism was investigated by thermogravimetric analysis(TGA). 29.93 mg of the gel polymer obtained from Example I was heatedunder flowing nitrogen at a heating rate of 10° C. per minute to 1100°C. The total weight loss was 17%. Results for the controlled pyrolysisare summarized in Table II below.

                  TABLE II                                                        ______________________________________                                        TGA Results for Pyrolysis of Polymer Gel                                      Temperature Range                                                                              Weight Loss                                                  ______________________________________                                         80° C.-430° C.                                                                  0.3%                                                         430° C.-605° C.                                                                  5.0%                                                         605° C.-690° C.                                                                  5.0%                                                         690° C.-745° C.                                                                  2.0%                                                         745° C.-785° C.                                                                  2.0%                                                         785° C.-950° C.                                                                  2.5%                                                         Total            16.8%                                                        ______________________________________                                    

The ceramic conversion occurred in the 430° C.-950° C. temperaturerange. The derivatives of the TGA curve indicated three major pyrolysismechanisms at 430°-700° C., 650°-800° C. and 780°-900° C. The thirdpyrolysis step accounted for ca. 15% of the total weight loss.

EXAMPLE X

10 ml of phenyltriethoxysilane was mixed with 10 ml of tetraethoxysilanein a beaker, 2.8 ml of 1.0M acetic acid was added, and the solution wasadjusted to about pH 1 by adding several drops of concentratedhydrochloric acid. The gel produced by this method was pyrolyzed innitrogen to 1200° C. at a heating rate of 200° C. per hour to give 66.1%of a foam material with a 22.6% carbon content by weight.Thermogravimetric analysis of the black glass foam was performed inflowing air at a heating rate of 20° C. per minute to 1150° C. andshowed a loss in weight of 20.38% which began at around 550° C. Thecolor of the sample turned white, indicating that the residue is silicaand the carbon is not oxidation resistant as would be expected if thecarbon was bound to the silicon structure rather than being present as amixture of graphite in silica.

A second example using 20 ml of phenyl triethoxysilane was reacted with4 ml of tetraethoxysilane as in the first example reported above to givea 66% yield of a porous product containing 35.0% carbon by weight.Thermogravimetric analysis of this product showed a weight loss of34.05%, again demonstrating that the carbon present is not resistant tooxidation at high temperatures as is the carbon present in the blackglass made in Examples I-VIII.

EXAMPLE XI

A sol mixture was prepared as described in Example I and silicon carbidewhiskers (Tateho) were suspended in said mixture by ultrasonic agitationfor from 15 to about 30 seconds resulting in a stable suspension of thewhiskers. Polymerization of the suspension occurred after 120 minutesheating at 50° C. affording a rigid 3.5 centimeter diameter compositecylinder with about 13% by weight whisker content. Pyrolysis of thiscylinder at 200° C. per hour up to a temperature of about 1200° C. gavea cylinder whose diameter had contracted by about 20%.

EXAMPLE XII

In like manner as described in Example I, a mixture was prepared andthen polymerized at about 90° C. in about 10-15 minutes under a pressureof 70 psi. Gelation took about 90 minutes if the temperature is loweredto 55° C. and the pressure is atmospheric. An increase in pressure thenallows higher temperature polymerization and affords much shorterpolymerization times. Under atmospheric pressure, the sol liquid with 90ppm Pt started to foam when gelation temperature was over 65° C.

EXAMPLE XIII

A mixture was prepared in like manner as described in Example I and 50weight percent of silicon carbide powder was added to said mixture.Polymerization occurred at 85° C. and 70 psi in 15 minutes withoutbubble formation. In atmospheric pressure operation it is not possibleto obtain bubble-free samples for filler loadings exceeding about 20% byweight filler since the filler acts to produce nucleate bubbling as thetemperature is raised.

EXAMPLE XIV

For purposes of this example and for use in the following ExamplesXV-XX, a standard mixture of cyclosiloxane monomers was prepared from T,tetravinyltetramethylcyclotetrasiloxane, and M, a commercial mixture ofmethylhydrocyclosiloxanes when the silicon atoms number from 4-6, in theratio of 5.7 T to 4.3M in the presence of 90 ppm platinum.

In this example 4 milliliters of the standard mixture was placed in apolypropylene tube and heated at 55° C. in an oven for 90 minutes toform a polymer which was subsequently hardened at 80° C. for 30 minutes.The polymer exhibited a smooth surface and no cracks after removal fromthe polypropylene tube. Pyrolysis of this polymer in nitrogen to 1200°C. at the rate of 200° C. per minute afforded a black glass with a bulkdensity of 2.05 g/ml exhibiting a diameter shrinkage of about 21% and areduction in volume to about 49% of initial volume.

EXAMPLE XV

Twelve milliliters of the mixture of Example XIV was mixed with 3 gramsof alpha silicon carbide powder with ultrasonic agitation and heated to40° C. for 15 minutes. The resultant mixture was poured into an 11 mm×11mm×55 mm copper case and placed in a pressure vessel under 90 psig. Thepressurized container was placed in a 60° C. water bath for 60 minutesto polymerize the monomers. The polymer was heated in a 90° C. oven forone hour, and then taken out of the copper case. The filled polymer haddimensions of 1.44 cm×1.44 cm×4.60 cm and exhibited a smooth surface andno cracks. Pyrolysis under nitrogen at 200° C. per hour up to 1200° C.produced a 1.14 cm×1.14 cm×3.60 cm black glass object with uniformshrinkage, as to height and width, of 19.0% and a final volume of 52.9%of the initial volume.

The black glass was then impregnated under vacuum with the startingmixture, polymerized at 55° C. for 80 minutes, cured at 90° C. for 60minutes, and pyrolyzed to 1200° C. as before. The pyrolyzed black glassexhibited no change in dimensions and weighed 10.28 grams with a densityof 2.3 g/ml. The silicon carbide was present in the black glass at 23%by weight.

EXAMPLE XVI

A comparison experiment was run as described in Example XV but without apressurized vessel. The resultant polymer exhibited gas pockets andcracks after curing at 55° C. for 90 minutes. A similar non-pressurizedsample was cured at 35° C. for 16 hours and exhibited cracks and gaspockets. When a similar mixture was cured at 22° C. for 48 hours, thepolymer did not exhibit cracks and gas pockets but sedimentation of thealpha silicon carbide powder occurred and resulted in a clearly definedboundary layer in the polymer.

EXAMPLE XVII

Eleven grams of a starting mixture as described in Example XIV was mixedwith 7 grams of alpha silicon carbide powder (39.3% by weight) byultrasonic dispersion, placed in a cylindrical aluminum case with aninterior diameter of 18 mm and a height of 74 mm, pressurized to 110psig, immersed in an 85° C. water bath for 15 minutes, heated in a 100°C. oven for one hour, and the filled polymer was then removed from thecase and exhibited a smooth surface having no cracks with a diameter of18 mm and height of 48 mm. Upon pyrolysis to 1200° C. at 200° C. perhour under nitrogen, the filled black glass exhibited a 15 mm diameterwith a height of 41 mm, a weight of 16.34 grams, a density of 2.2 g/ml,and contained 43% by weight of silicon carbide powder.

EXAMPLE XVIII

In like manner as in Example XVII, a 61% alpha silicon carbide filledmonomer mixture was prepared from 16 grams of alpha silicon carbidepowder and 10 ml of monomer mixture. This mixture was heated at 40° C.for 20 minutes, the slurry was then poured into an 11 mm interiordiameter polypropylene tube, the tube was pressurized to 110 psig andheated for 8 minutes at 80° C. and aged at 85° C. for 30 minutes, andthe polymer was removed and exhibited a smooth surface and no cracks.Pyrolysis to 1200° C. under nitrogen at 200° C. per hour gave a filledblack glass with a diameter of 8.7 mm and a height of 45.7 mm, a weightof 6 grams, a density of 2.23 g/ml, and contained 66% by weight ofsilicon carbide.

EXAMPLE XIX

In like manner as in Example XVIII, 14.5 ml of monomer mixture wascombined with 5.5 grams of silicon carbide whiskers and the whiskerswere dispersed ultrasonically. The slurry was then poured into a 58 mm×18mm×12 mm rectangular aluminum mold and polymerized at 50° C. for 3hours when the polymer was separated from the mold and exhibited cracksand gas pockets. Pyrolysis was performed at 200° C. per hour undernitrogen to 1200° C. resulting in a black glass with a 17.8 g (86.6%yield) having dimensions of 50 mm×15 mm×11 mm and a final volume of 67%of the initial volume. The density was 2.16 g/ml and 32% by weight ofthe filled black glass was silicon carbide whisker.

EXAMPLE XX

As described in Example XIX, the standard monomer mixture was heated at40° C. for 30 minutes and then 10 milliliters was mixed with 2.99 gramsof silicon carbide whiskers. This mixture was divided into two parts.One part was polymerized at 55° C. at atmospheric pressure for 90minutes and produced a polymer exhibiting gas pockets and surfacecracks. The second part was polymerized under 70 psig at 85° C. for 15minutes giving a polymer exhibiting a smooth surface and no cracks. Thisexperiment shows the importance of pressure in shortening the reactiontime and in producing a crack-free product.

EXAMPLE XXI

A sol solution, prepared as described in Example II was mixed withsilicon carbide fibers (Nicalon™- manufactured by Nippon) in a 1.9millimeter glass vial. The mixture was polymerized at 52° C. for abouttwo hours resulting in a crack-free fiber-reinforced polymer monolith.Controlled pyrolysis, wherein the temperature was raised 200° C. perhour until a final temperature of 1200° C. was reached, resulted in theformation of a black glass monolith with the same diameter as thepre-fired polymer monolith, a result that was not shown by theunreinforced polymer of Example II. The Nicalon fiber-reinforcedmonolith had a density of 1.0 grams per milliliter and contained about9.7% by weight silicon carbide fiber.

The black glass monolith was impregnated, under vacuum, with monomermixture and pyrolyzed in nitrogen to 1200° C. at 200° C. per hour togive a monolith with a density of 1.4 grams per milliliter. A secondimpregnation with monomer mixture followed by further pyrolysis gave ablack glass monolith with a density of 1.6 grams per milliliter.

EXAMPLES XXII

A cyclosiloxane where R and R' groups are only hydrogen, i.e.,1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane is reacted with acyclosiloxane where R and R' groups are partly vinyl, i.e.,tetravinyltetramethylcyclotetrasiloxane in the presence of platinumdivinyl tetramethyl disiloxane complex containing 3% platinum at atemperature of about 60° C. for one hour to form a polymer. The productis pyrolyzed in nitrogen to form a black glass as described above.

EXAMPLE XXIII

60.09 grams of hydromethylcyclosiloxane (mixture of trimer, tetramer,pentamer, hexamer and heptamer) (HMCS), 13.0 grams of acetylene and 0.10gram of chloroplatinic acid are charged into a pressurized reactor. Thereaction mixture is heated to 80° C. at a pressure of 500 psi for 5hours. The resulting gelled polymer is pyrolyzed in flowing air ornitrogen to about 1150° C. at 200° C. per hour to form black glass.

EXAMPLE XXIV

46.09 grams of dihydrocyclosiloxane (mixture of trimer, tetramer,pentamer, hexamer and heptamer) (DHCS), 26.0 grams of acetylene and 0.10gram of chloroplatinic acid are charged into a pressurized reactor. Thereaction mixture is heated to 80° C. at a pressure of 500 psi for 5hours. The resulting gelled polymer is pyrolyzed in flowing air ornitrogen to about 1150° C. at 200° C. per hour to form black glass.

EXAMPLE XXV NMR Data of Polymer Precursor

Solid state nuclear magnetic resonance (NMR) ¹³ C and ²⁹ Si spectra ofthe polymer precursor prepared as Polymer A in Example 1 were obtained.The Polymer A was further post-cured at 100° C. for 3 hours. The polymermass was ground into powder and packed tightly into a zirconia rotor.The NMR experiments were run at 30° C.

The NMR spectra were acquired with Chemagnetics CMX-300 solid state NMRspectrometer, operating at 75 MHz for ¹³ C and 59.5 MHz for ²⁹ Si. Thespectra were acquired via cross polarization. 5 μs 90° pulses wereapplied with 5 KHz magic angle spinning rate. The ²⁹ Si spectrum wasacquired with 10 ms contact times and 5 seconds cycle times while for ¹³C spectrum the contact time was 5 ms and the cycle time was 3 seconds.

The ²⁹ Si spectrum showed a major peak at -20 ppm which corresponded tothe (CH₃)SiO₂ (CH₂ CH₂) silicon configuration. A lesser peak was foundat -36 ppm corresponding to the (CH₃)SiO₂ (CHCH₃) configuration. Thestructures are formed from hydrosilylation between hydrosilane andvinylsilane moieties. There were three other minor peaks with chemicalshifts (σ), assignments and intensity percentages listed in thefollowing table. The peak at -11 ppm appears to represent one of themonomers. The peaks at -57 and -67 ppm were not found in the monomersand are considered to represent structures formed by rearrangementreactions occurring during polymerization at elevated temperatures.

    ______________________________________                                                 Assignment  Percentage                                               ______________________________________                                        -20        (CH.sub.3)SiO.sub.2 (CH.sub.2 CH.sub.2)                                                     76.7                                                 -36        (CH.sub.3)SiO.sub.2 (CHCH.sub.3)                                                            10.2                                                 -57        (CH.sub.3)SiO.sub.3                                                                         6.4                                                  -67        (CH.sub.2)SiO.sub.3                                                                         4.5                                                  -11        CH.sub.3 SiHO.sub.2                                                                         2.2                                                  ______________________________________                                    

The above results suggest that the silicon atoms are linked by ethylenegroups (σ=-20) and to a lesser extent by methylene groups (σ=-36). Thesecorrespond to reactions 3 and 4 described earlier and the alternativemoieties which have been described and are claimed below.

The ¹³ C NMR spectrum showed two major peaks which can be assigned to beSi-CH₃ and Si-CH₂ -. The data were summarized as follows:

    ______________________________________                                        σ, ppm Assignment Percentage                                            ______________________________________                                         4           Si--CH.sub.3                                                                             58                                                    12           Si--CH.sub.2 --                                                                          33                                                    19           SiCH(CH.sub.3)Si                                                                          5                                                    140          Si--CH═CH.sub.2                                                                       4                                                    ______________________________________                                    

The structure of the polymer precursor revealed from the carbon andsilicon NMR shows the existence of both methylene (σ=19) and ethylenelinkages (σ=12) between silicon atoms as expected from the results ofthe ²⁹ Si spectrum. The Si-CH₃ linkage is considered to be the samelinkage found in the two monomers, both of which contained methyl groupsattached to silicon atoms.

EXAMPLE XXVI ²⁹ Si NMR Data of the Silicon Carboxide

The black glass prepared in Example I was analyzed by solid state magicangle spinning nuclear magnetic resonance spectroscopy. The spectrum wasacquired at 59.497 MHz on a Chemagnetics CMX300 NMR spectrometer. Datawas obtained by single pulse technique without decoupling. Recycle timewas 6000 seconds with 400 scans. An external reference of3-(trimethylsilyl)-1-propanesulfonic acid, sodium salt, was used tocalibrate the chemical shift (1.18 ppm). The NMR peaks were deconvolutedusing NMRI computer software and a gaussian lineshape function wasemployed to resolve the peaks. The characters of the deconvoluted peakswere listed as follows:

    ______________________________________                                        σ, ppm  Assignment                                                                              Percentage                                            ______________________________________                                        -111          Si--O.sub.4                                                                             27.7                                                  -73           C--Si--O.sub.3                                                                          16.2                                                  -34           C.sub.2 --Si--O.sub.2                                                                   22.6                                                  -12           C.sub.4 --Si                                                                            33.5                                                  ______________________________________                                    

The NMR data demonstrates that the silicon is bonded to differentcombinations of silicon and oxygen atoms, forming a silicon oxycarbideor silicon carboxide network.

We claim as our invention:
 1. A black glass ceramic produced bypyrolyzing at a temperature of 800° C. to 1400° C. a precursor polymercontaining the moieties ##STR8## where R^(o) is the unreacted residue ofan alkene having 2 to 20 carbon atomsR' is H, an alkyl group having 1 to20 carbon atoms, or an alkene having 2 to 20 carbon atoms.
 2. A blackglass ceramic of claim 1 wherein said moiety is derived from thereaction of cyclosiloxane monomers containing silicon-vinyl bonds and/orsilicon-hydride bonds.
 3. A black glass ceramic of claim 1 wherein saidmoiety is derived from the reaction of cyclosiloxane monomers containingsilicon-hydride bonds with acetylene.
 4. A black glass ceramic of claim1 wherein said moiety is derived from the reaction of cyclosiloxanemonomers containing two hydride bonds attached to a silicon atom withcyclosiloxane monomers containing silicon-vinyl bonds.